Cordless telephone installations are commonly used to allow the user to communicate with the local telephone system by means of two-way radio communication units. Such systems typically consist of a base unit directly connected to the telephone lines and having a transmitter-receiver unit therein configured for two-channel duplex operation. The transmitter and receiver therein are capable of simultaneous operation with a pair of different frequencies in the 46-49 megahertz range. A battery-powered portable handset is also provided, similarly having a duplex transmitter-receiver therein, and tuned to the corresponding base frequencies. Each unit has associated therewith a transmitting/receiving antenna.
Conventionally such antennas are configured as extensible sub-wavelength whips mounted to extend well beyond their associated housings. Such antennas have certain disadvantages. First, they are readily broken or bent because of their inherently fragile structure. Also, many individuals prefer not to have such visible antenna structures in the home. It is therefore desirable that, if possible, the antenna be completely contained within the handset housing.
Two related problems immediately arise. If a very short whip antenna is employed, then in order to achieve even minimal radiation properties, the whip must extend the full length of the housing. Not only are the radiation properties unsatisfactory, but so extending the antenna into the portion of the housing where the user places his hand will give rise to unacceptable detuning effects, since the proximity of the hand provides additional capacitive loading on the whip.
Improved radiating properties can be secured by configuring the antenna as an inductive spiral, but even in this case a spiral wound antenna having adequate radiating properties must be sufficiently long to extend into the gripping region, again suffering from the above mentioned detuning effect. Attempts to overcome the foregoing problems by fashioning the antenna as a stripline consisting of zigzag metalizations on a printed circuit board proved similarly unsuccessful.
Prior to the instant invention there remained a need for an antenna having adequate radiating properties, and preferably configurable to have its major dimension no longer than approximately 1/100 of a wavelength so to enable it to be placed remote from the gripping portion, and further having sufficient capacitance that proximity effects, either from the user's hand or the closeness of the user's face during use do not introduce unacceptable detuning.